The present invention relates to a method of preparing brine by dissolving salt containing calcium sulfate or other impurities. The uptake of the impurities into the brine is reduced by supplying solid salt and water to a vessel including a grid for supporting a salt bed. The water flows through and dissolves the sodium chloride portion of the salt, while a great deal of the impurities pass through the grid whereafter they are removed from the solution. The invention also concerns a device for preparing brine by the method described.
Brine, i.e. an aqueous solution of sodium chloride, is used as a raw material in several electrochemical processes, such as production of sodium chlorate. The brine is often prepared by dissolving rock salt in water. Even if rock salt mainly consists of sodium chloride, it generally contains considerable amounts of other salts, such as calcium sulfate. One kg rock salt often contains as much as 20 g calcium. Calcium sulfate, as well as some other impurities, are less soluble than sodium chloride. It is desirable to obtain brine that is as concentrated as possible in respect of sodium chloride, preferably saturated.
In conventional processes for preparing brine, the water flows through a salt bed in a dissolving zone. Some of the calcium sulfate is dissolved and included in the brine, while some is set free and accumulates in the dissolving zone. Such processes involve two main problems.
Firstly, the brine produced contains a considerable amount of calcium sulfate, generally more than 400 mg calcium per liter. Especially the calcium must be removed before the brine is used in a chlorate process, which is done by reacting with soda ash. It is also desirable that the content of sulfate is as low as possible.
Secondly, the calcium sulfate as well as other impurities which is not dissolved accumulates in the dissolving zone and thus decreases the amount of sodium chloride available for dissolving. In other words, the contact surface between the sodium chloride and the water is decreasing and the contact surface between the impurities and the water is increasing during operation. After some time of operation, the brine produced will therefore be less concentrated in respect of sodium chloride but having a higher content of impurities, such as calcium sulfate. The dissolving device must then be taken out of operation and cleaned from calcium sulfate and other accumulated impurities.